A light-sensitive material having a photo-conductive layer which is used for making a printing plate by an electrophotographic method is known. For example, a light-sensitive material used for a printing plate comprising a support having provided thereon a metal layer, a photoresist layer and a photoconductive layer has been proposed.
Complicated steps which comprise uniformly charging a photoconductive layer, imagewise exposing it to a light to which a photoresist layer is not light-sensitive to prepare an electrostatic latent image on the photoconductive layer, developing the latent image with a toner, fixing or non-fixing a toner image, exposing the photoresist layer under a different condition from that as mentioned above to harden the exposed portion of the photoresist layer, removing the toner image and the photoconductive layer, removing the unhardened portion of the photoresist layer with a solvent, etching a metal layer and removing the photo-resist layer remaining on a metal layer are required to prepare a printing plate using the above mentioned light-sensitive material. A high degree of skill is required to prepare a good printing plate. However, the resolving power of the plate is not satisfactory since the developing method of an electrostatic latent image formed on the photoconductive layer is formed by a dry method. Furthermore, clearness of a printing image thus obtained is not satisfactory because of the removal of a photo-resist layer and an etching operation of a metal layer.
To eliminate the above defects, European Patent Application, Published No. 0053362 discloses a light-sensitive material which comprises an electroconductive support having a hydrophilic surface, a positive working light-sensitive layer and a photoconductive insulating layer wherein the positive working light-sensitive layer and the photoconductive insulating layer form one layer or the former and the latter are provided on the support respectively. The photoconductive insulating layer is substantially capable of being charged negatively or positively and does not prevent the selective removal of the positive working light-sensitive layer. The applicant of the present invention also proposed a method for lithographic printing using the above light-sensitive material which comprises (1) a step of forming a latent image electrophotographically on a photoconductive layer of the light-sensitive material, (2) a step of developing the latent image with a liquid developer containing developer particles which are opaque to the light to which the positive working light-sensitive layer is light-sensitive, (3) a step of exposing the positive working light-sensitive layer through the developed image thus obtained in the step (2), and (4) a step of selectively removing the areas of the positive working light-sensitive layer which do not have the developed image. The above method and the light-sensitive material are disclosed and claimed in European Patent Application Published No. 0053362.
The above method offers an improvement on the conventional method with respect to simplification of the steps and resolving power. However, a printing stain is caused when printing is carried out using the printing plate obtained by the above method.
The inventor of this invention has investigated the cause of the stain and has found that it was caused because, where a photoconductive insulating layer is charged and is imagewise exposed to form an electrostatic latent image, the electric charge on the exposed areas is not completely removed, a toner is attached to the exposed areas at the development, and where the positive working light-sensitive layer is exposed, unexposed areas remain because of the toner attached to the non-imaged areas as a fog in the step (3), and when the light-sensitive layer is selectively removed in the step (4), the unexposed areas remain on a support.
The inventor has found that the printing stains are caused by the presence of the aluminum oxide formed on an aluminum or an aluminum alloy support and can be prevented where the aluminum oxide layer is kept in the amount of 0.2 to 2.8 g/m.sup.2, and has filed as a co-application Japanese Patent Application No. 32214/82.